Supercontinuum radiation is a phenomenon whereby narrowband light propagating through a non-linear medium is transformed into a signal with very broad spectrum. Optical fibers are a natural choice of non-linear media.
J. Swiderski and M. Michalska, “Mid-infrared supercontinuum generation in a single-mode thulium-doped fiber amplifier” Laser Phys. Lett. 10 (2013) 035105 (6 pp), discloses a supercontinuum source comprising thulium-doped fibers. The wavelength of the output spectrum of the supercontinuum source extends from 1.75 to 2.7 μm. The method disclosed therein is as follows. Firstly, nanosecond seed pulses are amplified in a cascade of erbium and erbium:ytterbium fiber amplifiers. The amplified pulses are then passed into a single-mode fiber and are shifted to about 2.4 μm using soliton self-frequency shifting. The spectrum is extended above 2.4 μm by using a core-pumped thulium-doped fiber amplifier, and that radiation is then amplified in another section of thulium-doped fiber amplifier to produce the output supercontinuum. Increasing the pump power broadens the spectrum of the supercontinuum.
It will be appreciated that the above system is rather complex, and it would thus be advantageous to provide a simpler and more versatile fiber amplifier.